Keep Fastly identity tests off the network and behind a resolvable origin #1560

Follow-up from PR #1487 (review round 3).

Description

The page-phase identity test in crates/trusted-server-adapter-fastly sends a real provider lookup during routing. Dispatch uses FastlyPlatformHttpClient; the counting test client only replaces it at collect. Viceroy logs a DNS error for the fictional provider host (ids.example.com), and no assertion checks the send count, so the test passes whether or not the lookup was sent.

All the Viceroy identity tests also run behind a Trusted-Server-generated 502, because the fixture origin does not resolve. They therefore exercise the origin-unreachable path rather than a normal page response.

Done when

• The lookup client is injected at dispatch, so the test client sees every send and no real network request leaves the test.
• Each identity test asserts the expected provider send count.
• The Viceroy fixture has a resolvable origin (for example a local backend), so identity tests run against a normal origin response; the 502 path is covered by an explicit test where intended.
• Viceroy test logs show no DNS errors for provider or origin hosts.

Affected area

Fastly runtime

Code: crates/trusted-server-adapter-fastly (identity Viceroy tests, FastlyPlatformHttpClient dispatch path).